Composites incorporated a conductive polymer nanofiber network

ABSTRACT

Methods of forming composites that incorporate networks of conductive polymer nanofibers are provided. Networks of less-than conductive polymers are first formed and then doped with a chemical dopant to provide networks of conductive polymers. The networks of conductive polymers are then incorporated into a matrix in order to improve the conductivity of the matrix. The formed composites are useful as conductive coatings for applications including electromagnetic energy management on exterior surfaces of vehicles.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/618,126, filed Mar. 30, 2012, the disclosure of which is herebyincorporated by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with government support under contract numberDESC0005153, awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

BACKGROUND

Carbon fiber reinforced composites (CFRC) result in significantreductions in weight due to their lower density (1.5-1.8 g/cm³) whencompared to aluminum alloys (2.7 g/cm³). CFRC are particularly ofinterest in the manufacture of vehicles, including airplanes and landvehicles such as cars and trucks. Furthermore, the use of compositessimplifies manufacturing processes and leads to stronger components withhigher durability. Expected increases in fuel costs will continue tomotivate the use of composites in vehicle designs in the future.

Despite having clear advantages in weight reduction and fuel savings,the use of resin-based composites in modern vehicles presents newengineering challenges. Some of these stem from the dielectric nature ofthe resins that are used to prepare the CFRC. Traditional metalliccomponents have high conductivity (Aluminum ˜3.6×10⁷ S/m). Carbon fibersin CFRC significantly increase the effective conductivity (CFRC ˜1.5×10⁴S/m) but, because they are buried inside the material, large currentscan cause structural damage. For these reasons, electromagnetic effect(EME) management of CFRC in vehicles (particularly airplanes) isespecially important.

In particular, the mitigation of risks associated with lightning strikesis of high relevance to aircraft design. It is estimated thatFAA-approved commercial airplanes are struck by lightning an average oftwo times every year. The primary lightning event (main stroke) requiresdissipation of up to 200,000 Amperes over sub-millisecond timescales.When a suitable conductive path is not present, mechanical damage,thermal degradation and/or damage to electronic components can result.Moreover, lightning-related events such as corona discharge, streamers,and continuing currents can also persist before or after the main strokeexits the airplane. These events can result in serious damage tophysical and electronic components even when not in the exit path of themain stroke. For example, continuing currents can also be significant(up to 200 Amperes) and need to be dissipated effectively.

The other main EME problem of importance is static charge buildup duringnormal flight conditions. Static charge can originate from the impact ofairborne particles, rain or snow (i.e. triboelectric charging) or fromthe flow of hydraulic fluids or fuel. Static charge buildup hinderscommunications, interferes with electronic equipment and can lead tosparks and explosions in the presence of flammable vapors. The increaseduse of electronic navigation systems in modern aircrafts (e.g.fly-by-wire) that may be affected by static buildup further motivatesthe need for effective charge dissipation.

Current strategies for lightning and EME management in planes containingCFRCs consist of incorporating a conductive metallic mesh (e.g. Cu orAl) between upper plies of the composite. This allows effective currentdissipation along the surface of the plane without penetrating deep intothe composite material. Although this is an effective damage preventionstrategy, it can add significant weight to the plane (Cu density is 8.9g/cm³), reducing the magnitude of fuel savings. For this reason,metallic meshes are only added to critical sections of the planes suchas those with high probability of lightning strike or where damage canbe critical (e.g. fuel tanks).

A more powerful mitigation strategy that is also being explored is theuse of conductive finishes (i.e. coatings) on the upper surfaces of theplane.

Because of their location, sacrificial conductive coatings canpotentially dissipate enough electric current to prevent more seriousdamage to underlying structural and electronic components. Althoughlightning will irreversibly damage the coatings, these can be easilyremoved and reapplied. In contrast, damage to composite parts requiresfull replacement of the affected area at a much higher cost. Currentcommercial conductive finishes are usually composed of silver or copperparticles dispersed within epoxy, acrylic or polyurethane carriers. Theuse of metallic particles leads to low sheet resistances (˜0.1 Ohm/sqfor 0.05 mm thickness) but the creation of a connected conductive path(percolation) requires very high particle loadings (>50 wt %). This alsotranslates to very large mass densities (>4 g/cm³) for the resultingcoatings, adding to the total aircraft weight and reducing fuel savings.More importantly, the high particle loading requirements significantlydeteriorate the mechanical and adhesive properties of the coatings sothat they may not meet aerospace requirements.

Ideally, one would create conductive finishes that have low sheetresistances, low mass densities and which do not affect the adhesive andmechanical properties of existing coatings that have been optimized forthis application. Conductive nanomaterials have been proposed aspossible conductive finishes. The dispersion of conductive nanomaterialsincluding carbon nanotubes, graphene, and nanoparticles into organicresins has been explored in order to modify the electronic properties ofcomposite materials. Although some of these strategies show substantialpromise, there are also significant problems preventing theirapplication in conductive finishes. For example, carbon nanotubes havelow percolation thresholds (˜0.5 wt %) and show significant increases inconductivity at higher concentrations (e.g. ˜0.2 S/m at 1 wt %).However, these changes are also followed by large increases in viscositythat makes coating difficult. There are also concerns about the toxicityof nanotubes and the potential for stronger regulation in the future.

Therefore, improved conductive finishes are desirable in order toadvance the production of CFRCs and similar technologies.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, a method of forming a composite incorporating networks ofconductive polymer nanofibers is provided. In one embodiment, the methodincludes the steps of:

(a) providing a colloidal dispersion comprising a self-assembled networkof a conjugated polymer;

(b) doping the conjugated polymer with a chemical dopant to provideconductive polymers within the self-assembled network of the colloidaldispersion; and

(c) dispersing the colloidal dispersion within a liquid matrix toprovide a liquid composite comprising a network of conjugated polymernanofibers, wherein the liquid matrix is selected from the groupconsisting of a polymer and a polymer precursor.

In another aspect, an electromagnetic effect (EME) management system fora vehicle exterior is provided. In one embodiment, the EME managementsystem includes:

-   -   an exterior surface of a vehicle; and    -   a composite layer disposed on the exterior surface, the        composite layer comprising a network of conductive polymer        nanofibers in a solid polymer matrix.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1D schematically illustrate representative methods of formingcomposites incorporating polymer nanofiber networks in accordance withthe disclosed embodiments.

FIG. 1E schematically illustrates the effect of chemical doping ofconjugated polymers on nanofiber networks formed before and afterdoping.

FIG. 2 illustrates representative conjugated polymers useful in thedisclosed embodiments.

FIG. 3 illustrates TEM micrographs of P3HT nanofiber networksself-assembled in various solvents.

FIG. 4 illustrates the formation of stable networks of P3HT in epoxy(BADGE) in accordance with the disclosed embodiments.

FIG. 5 illustrates TEM micrographs of nanostructured P3HT formed inxylene through self-assembly triggered by a temperature change from 80°C. to −20° C.

FIG. 6 illustrates SEM micrographs of deposited colloidal particlesafter doping conjugated polymer nanofiber networks contained thereinwith iodine.

FIG. 7 graphically illustrates small angle x-ray scattering (SAXS)profiles of P3HT self-assembled in toluene at a concentration of 0.2 wt% and at a temperature of −20° C. after dissolution at 80° C.

FIG. 8 graphically illustrates bulk conductivity of nanostructured P3HTcolloidal networks, formed in xylene through a temperature change, as afunction of concentration and doping (excess iodine). Percolationthresholds are in the range of 0.3 to 0.6 wt % filler.

FIG. 9 graphically illustrates bulk conductivity of a P3HT dispersiondoped with iodine (in excess) and dispersed in pure epoxy precursor(BADGE or bisphenol A diglycidyl ether) as a function of shear rate.Measurement is performed in a rheometer with 25 mm parallel plates.Conductivity improves or is maintained during shear flow up to shearrates of 10 s⁻¹. Also illustrated is a schematic and picture of therheo-dielectric testing apparatus.

FIG. 10 illustrates images of a film of P3HT doped with iodine afterself-assembly and dispersed at a concentration of 1.2 wt % in acommercial polyurethane formulation. The conductive film is flexible andretains the properties of pure polyurethane films because of the lowfiller loading fractions.

FIG. 11 illustrates SEM micrographs of common nanostructures formed fromconjugated polymers.

FIG. 12 is a circuit model and Nyquist plot of the impedance of a dopedP3HT network in uncured epoxy.

DETAILED DESCRIPTION

Methods of forming composites that incorporate networks of conductivepolymer nanofibers are provided. Networks of less-than conductivepolymers are first formed and then doped with a chemical dopant toprovide networks of conductive polymers. The networks of conductivepolymers are then incorporated into a matrix in order to improve theconductivity of the matrix.

In one aspect, a method of forming a composite incorporating networks ofconductive polymer nanofibers is provided. In one embodiment, the methodincludes the steps of:

(a) providing a colloidal dispersion comprising a self-assembled networkof a conjugated polymer;

(b) doping the conjugated polymer with a chemical dopant to provideconductive polymers within the self-assembled network of the colloidaldispersion; and

(c) dispersing the colloidal dispersion within a liquid matrix toprovide a liquid composite comprising a network of conjugated polymernanofibers, wherein the liquid matrix is selected from the groupconsisting of a polymer and a polymer precursor.

The disclosed methods may be better understood with reference to theexperimental descriptions below, as well as the attached FIGURES.Referring to FIGS. 1A-1C, the steps of the method will be described.

FIG. 1A illustrates a colloidal dispersion 100 comprising aself-assembled network of conjugated polymer nanofibers 105 in a solvent110. The composition of the colloidal dispersion 100 will be describedin greater detail below.

“Colloidal dispersion” is defined as a conjugated polymer networkstructure that is suspended in a solvent. This suspended structure inits entirety can be on the order of 100 nm or larger, but may containcomponents that exist on a much smaller size scale (i.e. some conjugatedpolymers form fiber that are on the order of 5 nm in thickness and 20 nmin width). The smaller, interconnected components make up the largernetwork structure that is suspended in the solvent.

Referring to FIG. 1B, the colloidal dispersion 100 of FIG. 1A is thendoped with a chemical dopant 115, which has the effect of increasing theconductivity of the nanofibers and the colloidal dispersion 100.

Referring to FIG. 1C, the colloidal dispersion 100 of FIG. 1B is thentransferred to and dispersed in a liquid matrix 125 to provide a liquidcomposite 120 comprised of a network of conductive polymer nanofibers.The liquid matrix 125 includes a polymer or a polymer precursor.

In one embodiment, the network of conductive polymers comprises fibershaving an individual length of from 50 nm to 5 microns.

In one embodiment, the network of conductive polymers comprises fibershaving a cross-sectional dimension of from 5 nm to 200 nm.

In one embodiment, the network of conductive polymers comprises fibershaving a plurality of branch points spaced between 200 nm to 5 micronsapart. As used herein, “branch points” refers to the locations along apolymer chain 105 where the polymer branches into side chains. Referringto FIG. 1A, branches are illustrated at 107 and 109. The distancebetween branch points is the distance between 107 and 109.

In one embodiment, the colloidal dispersion is from 1 micron to 1 mm insize. The size of the colloidal dispersion is defined by its largestmeasurable dimension (e.g., width). The shape of typically colloidaldispersions is irregular.

In one embodiment, the method further comprises a step of solidifyingthe liquid composite to provide a solid composite comprising a networkof conjugated polymer nanofibers in a solid polymer matrix. Referring toFIG. 1D, the liquid composite 120 of FIG. 1C can then be applied to asubstrate 215 in order to form a solid composite 205. The solidcomposite 205 includes a solid polymer matrix 210 that is the solidifiedembodiment of the liquid matrix 125 from FIG. 1C (e.g., a polymerizedpolymer precursor or a solidified polymer). The solid composite 205 alsoincludes conductive polymers comprising a conjugated polymer nanofibernetwork 105 doped with chemical dopants 115. The combined assembly 200provides a conductive surface to the substrate 215 that includes aplurality of conductive paths, via the nanofiber network 105 from thesubstrate surface 220 to the composite surface 225.

In the representative embodiment where the substrate 215 is a vehicleexterior surface, the solid composite 205 provides an EME managementlayer. Representative substrates 215 include non-conductive structuralmaterials, such as CRFCs. Representative vehicles include airplanes andautomobiles (e.g., cars, trucks, and motorcycles).

The disclosed embodiments use conjugated polymer nanostructures asadditives to generate finishes for electromagnetic effect (EME)management applications on carbon fiber reinforced composites (CFRC).Conjugated polymers (CP) have delocalized electrons in π orbitals alongthe backbone. Charge transport can occur along the chain by resonanttransfer or via inter-chain “hopping” when polymers are packedsufficiently close to each other (e.g. via π-π stacking in nanofibers).

Conjugated polymers are not considered “conductive polymers” for thepurposes of this disclosure, unless doped with a chemical dopant. Inthis regard, “conductive polymers” incorporated into a nanofiber networkand including the chemical dopant, have a conductivity of 10⁻⁹ S/sq orgreater (i.e., 10⁹ Ohm/sq or less).

Representative CPs include polythiophenes, polyfluorenes, polyacetylene,polyanililine and polyphenylenes with various substitution moieties(e.g. alkanes) that are added to improve solubility in organic solvents.In one embodiment, the conjugated polymer is a semiconducting polymer.Conjugated polymers by themselves are typically, at the most,semiconducting. Therefore, conjugated polymers must be doped, asprovided herein, in order to become sufficiently conductive to form aconductive composite.

In one embodiment, the conjugated polymer is organic-soluble. As usedherein, the term “organic soluble” refers to a conjugated polymer thatcan dissolve in an organic solvent at concentrations that are equal orgreater than 0.1 mg/mL.

In one embodiment, the conjugated polymer is selected from the groupconsisting of a polyalkylthiophene, a polydi-alkyl fluorene, apolydithienosilole, a polyphenylene, a poly(3,4-ethylenedioxythiophene),a poly(pyrrole), a polypyrene, a polypyridine, a poly(p-phenylenevinylene), a polycarbazole, a polyaniline, a polyindole, and a copolymerof the polymers listed within this group.

FIG. 2 shows chemical structures of two representative CP types usefulin the disclosed embodiments: polythiophene (e.g. P3HT) and polyfluorene(e.g. PFO).

Although largely unexplored in this context, CPs have potentialadvantages for the creation of advanced conductive finishes for CFRC invarious applications, including the aerospace industry. First, theirdensity (˜1.1 g/cm³) matches that of the resins so that they do not addsignificant weight to the coating. Their chemical nature also results infavorable chemical interactions with resin monomers so that they arestable in dispersion.

CPs useful in the provided embodiments possess strong tendencies toself-assemble into long nanofibers and to form networked nanostructures.The self-assembly process is usually triggered by the reduction of thepolymer solubility and it is readily controlled by changing temperatureor by adding miscible non-solvents. The formation of thesesupra-molecular nanostructures through self-assembly leads to sufficientelectronic percolation and to effective charge propagation.

Several CPs, including P3HT and PFO have been shown to readily formstable nanofiber network dispersions and organogels when dissolved inaromatic solvents at variable temperatures. Nanofiber networks andorganogels provide clear paths for charge transport over long distancesbecause they are intrinsically connected. Furthermore, inducingself-assembly under different conditions readily modifies the structuralparameters of the nanofiber network including the branching density(FIG. 3).

Network structures and gels have also been observed in several other CPsystems suggesting that it is a common effect that originates from π-πstacking interactions. The formation of self-assembled fibers andnetworks can result in multiple-order increases in conductivity due toincreased conjugation length, crystalline order and percolation.

In one embodiment, the colloidal dispersion is formed bytemperature-induced self-assembly of the conjugated polymer in asolution.

In one embodiment, the colloidal dispersion is a fluid colloidaldispersion.

In one embodiment, the colloidal dispersion is prepared from themechanical fracture of a gel.

In one embodiment, the gel is an elastic organogel comprising theself-assembled network of the conjugated polymer.

There is a strong correlation between the state of organization of theCPs and their electronic properties. Lastly, because they are normallyp-type semiconductors, the intrinsic conductivity of CPs is initiallylow but can increase by multiple orders of magnitude upon oxidativedoping. Conductivities of up to 20 kS/cm have been reported for dopedpolyacetylene.

FIG. 4 schematically shows a representative approach useful to generateall-organic conductive coatings. FIG. 4 also shows P3HT nanofibernetwork dispersions in epoxy carriers and cured coatings.

In certain embodiments, the disclosed composites are useful for thedissipation of currents arising from EME events related to static chargebuildup and lightning (e.g. corona discharge, streamers and continuingcurrents). One advantage of these conductive additives is that they canbe easily incorporated into CFRC finishes of vehicles (e.g., airplanes)so that most exposed areas of the vehicle could benefit from EMEprotection. In one embodiment, the liquid matrix is selected from thegroup consisting of a polymerizable resin, an oil-based paint, and anoil-based primer.

Accordingly, in another aspect, an electromagnetic effect (EME)management system for a vehicle exterior is provided. In one embodiment,the EME management system includes:

-   -   an exterior surface of a vehicle; and    -   a composite layer disposed on the exterior surface, the        composite layer comprising a network of conductive polymer        nanofibers in a solid polymer matrix.

In one embodiment, the solid polymer matrix is configured to be appliedto the exterior surface as a liquid coating.

In certain embodiments, the CP nanostructures can be added directly toexisting formulations (e.g. polyurethanes or epoxies) that have beenoptimized for mechanical properties, adhesion, durability, and cure-timeso that commercial implementation is straightforward.

Some CPs formulations, such as poly(3,4-ethylenedioxythiophene) dopedwith poly(styrene sulfonate) (PEDOT:PSS; BaytronP from Bayer) have beenused in antistatic applications and in transparent coatings for organicsolar cells and electronic displays. However, these coatings are usuallycomposed of pure PEDOT:PSS so that film properties (e.g. adhesion) arelargely determined by the CP and are not adequate for vehicle finishes.PEDOT:PSS is also insoluble in most solvents and application is normallyin the form of spherical particle dispersions where there is littleability to optimize the morphology and where high weight fractions arenecessary to achieve percolation. In contrast, polythiophenes (e.g.P3HT) and organic-soluble CPs show rich structural behavior that cantranslate into significant improvements in electronic properties (e.g.nanofiber networks shown in FIG. 3).

Procedures for Making Representative Composites

Step 1: Preparation of Fiber Network Dispersions from ConjugatedPolymers

Scheme 1: Colloidal Dispersions Self-Assembled Using Temperature Change

First, a conjugated polymer such as, poly-3-alkyl-thiophene (e.g.,poly-3-hexyl-thiophene; P3HT), poly di-alkyl fluorene, polydithienosilole vinylene, or poly dithienosilole thiazolothiazolethiophene, is dissolved in an organic solvent that can be composed of apure aromatic molecule (e.g. xylene, toluene, or benzene), an alkane(e.g. decane, dodecane, or hexadecane), a halogenated molecule (e.g.dichlorobenzene, chloroform), or a mixture of one or more of thesemolecules. Because conjugated polymers have limited solubility, thesamples must often be dissolved at a high temperature (e.g. >80° C. forP3HT in xylene or >120° C. for P3HT in dodecane). The totalconcentration of polymer in the solution is selected to be low(typically <1 wt %) to prevent the formation of an elastic gel when thetemperature is lowered. For P3HT in xylene, the gel point is about 0.5wt %.

Next, the temperature of the hot dissolved polymer solution is loweredto a value that induces self-assembly. For P3HT in xylene, this value isusually <60° C. The temperature that is used to induce polymerself-assembly will vary depending on the specific type of polymer andsolvent that is being used. The temperature can be lowered rapidly andheld fixed at a particular value or it can be lowered gradually using atemperature ramp. Depending on the temperature, solvent, polymer typeand concentration that is used, the final structure of theself-assembled polymer can be manipulated.

Next, the sample is allowed to undergo self-assembly for a totalduration that can range from a few minutes to several days. The resultis a stable and fluid colloidal dispersion of conjugated polymernanofiber networks such as those shown in FIG. 5. The size anddimensions of the dispersed fibers and the networks is controllable bymodifications to the polymer chemistry (monomer type), the molecularweight, the self-assembly temperature and the solvent.

In one embodiment, the colloidal dispersion is formed by self-assemblythrough the gradual change of solvent composition selected from thegroup consisting of alkanes, aromatics, and halogenated organicmolecules.

Scheme 2: Colloidal Dispersions from Organogel Self-Assembly andFragmentation

The first step is identical to the first step of Scheme 1 above, withthe following exception. The total concentration of polymer in thesolution is selected to be relatively high (typically >1 wt %) to forman elastic gel when the temperature is lowered and self-assembly occurs.For P3HT in xylene, the gel point is about 0.5 wt %.

Next, a step identical to the second step of Scheme 1 is performed, buta gel is formed instead of a fluid colloidal dispersion, due to the highpolymer concentration in solution.

Next, the sample is allowed to undergo self-assembly for a totalduration that can range from a few minutes to several days. The resultis an elastic conjugated polymer nanofiber network or organogel.

The solvent that was used to form the gel network in the previous stepscan be replaced (if desired) with a different solvent by adding the newsolvent to the top of the sample and allowing for diffusion to occur.The new solvent is chosen to not dissolve the self-assembled polymernetwork. Several consecutive changes of this solvent “cap” can be usedto fully replace the original solvent.

Finally, the gel sample is fractured mechanically. The method of gelfracture can be manual mixing, high-shear mixing or compounding,ultrasound fragmentation, extrusion or any other mechanical mechanism.The final particle size is determined by the method used to fragment thegels.

The fragmented gels result in a colloidal dispersion, similar to that ofScheme 1. The final concentration of the dispersion can be reduced byadding an adequate amount of solvent before the fragmentation process isinitiated. The result is a stable colloidal dispersion of nanofibernetworks.

Step 2: Chemical Doping of Network Dispersions

The nanostructures are chemically doped after inducing self-assembly andformation of fiber networks in Step 1.

FIG. 1E illustrates the issues associated with chemical doping when itis performed before self-assembly. Because doping results in theformation of a strongly associated anion-cation pair, the new polymerstructure is altered and self-assembly is prevented. The structure andconductivity of samples doped before self-assembly differs from that ofsamples that are doped after self-assembly, indicating the importance ofmaintaining structural control.

The following scheme is used to dope colloidal dispersions or gelsprepared as described in Step 1.

Typical doping molecules for conjugated polymers (p-type) are smallmolecules (e.g. iodine), organic soluble sulfonic acids (e.g.dodecyl-benzene sulfonic acid or DBSA) or ionic polymers (e.g. partlysulfonated polystyrene). The doping molecules are added directly to theconjugated polymer colloidal dispersions at a specified molar ratio withrespect to the total number of monomers present in the conjugatedpolymer sample. For this application, larger doping molecules (e.g. DBSAor sulfonated polystyrene) are typically used because smaller molecules(e.g. iodine) can slowly leach out from the self-assembled conjugatedpolymer structure and this can reduce the electrical conductivity overtime. Colloidal stability is maintained after doping, but sonication ormechanical agitation can also be used to increase dispersion quality.

In one embodiment, the chemical dopant is selected from the groupconsisting of oxidizing agents including iodine, organic solublesulfonic acids (e.g., dodecyl benzyl sulfonic acid), water-solublesulfonic acids (e.g., p-toluene sulfonic acid), organic salts (e.g.,iron III tosylate), and acidic polymers (e.g., polystyrene sulfonate inacid form).

FIG. 6 shows SEM images of deposited colloidal particles (produced usingStep 1, Scheme 1) after doping with iodine in excess after inducingself-assembly in xylene at 20° C. The large colloidal particles arevisible but the individual fibers are too small to resolve.

FIG. 7 and Table 1 show the results of small angle X-ray scattering(SAXS) experiments demonstrating that the fiber structure is preservedwhen doping is performed after self-assembly, but it is significantlyaffected when it is performed before self-assembly. The samples of FIG.7 are P3HT self-assembled in toluene at a concentration of 0.2 wt % andat a temperature of −20° C. after dissolution at 80° C. The SAXS modelfit in Table 1 demonstrates that fiber structure (thickness) ispreserved when nanostructures are doped with iodine after self-assembly.When doping is performed before self-assembly the fibers are thinner andnarrower.

TABLE 1 Results of SAXS fits to rectangular fiber model for P3HT self-assembled in toluene with I₂ dopant added before and after assembly.Post-Assembly Pre-Assembly Non-Doped Doped Doped Fiber Height (nm)  7.7± 0.2  7.8 ± 0.4  6.6 ± 0.5 Fiber Width (nm) 16.1 ± 0.6 21.0 ± 1.8 12.6± 0.9

The extent of doping can also be monitored and optimized by measuringthe bulk conductivity of the dispersions (FIG. 8). FIG. 8 illustratesbulk conductivity of nanostructured P3HT colloidal networks, formed inxylene through a temperature change, as a function of concentration anddoping (excess iodine). Percolation thresholds are in the range of 0.3to 0.6 wt % filler. Chemical doping is associated with a sharpenhancement of conductivity, where increases by factors of 10-1,000 ormore are typical.

Step 3: Dispersion of Doped Conjugated Polymer Nanostructures in Paints,Primer or Organic Resins.

Samples prepared via Steps 1 and 2 are dispersed into a polymerizableresin (e.g. epoxy BADGE) or an oil-based paint or primer formulation(e.g. polyurethane) by following these steps. Other possible matrixmaterials include polysiloxanes, acrylics, laquers, shellacs, alkyds,phenolic resins, dissolved polymers (e.g. polystyrene or polybutadiene)as well as polymerizable monomers such as styrene.

First, the doped conjugated polymer nanostructures are prepared bydispersing in a solvent that is miscible with the binder, paint orprimer that will be modified (made conductive). This can be achieved bycentrifugation, filtration or evaporation of the original solvent of theconjugated polymer nanostructure followed by the addition of the desiredamount of miscible solvent.

To achieve total re-dispersion of the conductive network, it is optionalto apply some external mechanical force via ultrasound or high-shearmixing.

The first steps can also be used to increase the concentration of theconductive filler (the conjugated polymer nanostructure) viare-dispersion in a smaller quantity of new solvent. Total removal ofsolvent should be avoided because it can cause the irreversible collapseof the colloidal network particles and a more compact structure willresult in larger percolation thresholds and lower conductivity. Also, itis desirable to concentrate the colloidal networks as much as possibleso that the addition of the additive does not cause the excessivedilution of the carrier paint or resin. Thus, samples should beconcentrated as much as possible while maintaining colloidal stabilityand allowing for homogeneous dispersion in the carrier. This is theoptimum formulation. Conditions will vary for different polymers,dopants and colloidal network structures.

In one embodiment, the step of dispersing the colloidal dispersionwithin the liquid matrix comprises dilution of the matrix and colloidaldispersion with a volatile organic solvent followed by concentration viasolvent evaporation using heat or vacuum.

In one embodiment, the step of dispersing the colloidal dispersionwithin the liquid matrix comprises sonication or mechanical blending.

After the filler additive is available in a miscible solvent at thedesired concentration, it is added to the carrier and mixed thoroughlyto ensure homogeneous dispersion. If the addition of the filler causesunwanted dilution of the carrier, the original concentration can beobtained through the evaporation of the solvent.

The dispersion of nanostructured conductive fillers in epoxy resins isdemonstrated in FIG. 9, which illustrates bulk conductivity of P3HTdispersion doped with iodine (in excess) and dispersed in pure epoxyprecursor (BADGE or bisphenol A diglycidyl ether) as a function of shearrate. Measurement is performed in a rheometer with 25 mm parallelplates. Conductivity improves or is maintained during shear flow up toshear rates of 10 s⁻¹.

The conductivity of the carrier BADGE by itself is only 0.01 S/m. When,1 wt % of P3HT nanostructures doped with iodine are added, theconductivity increases to a rest value (zero-shear) of 56 S/m.

Furthermore, when shear is applied, the conductivity increases to amaximum value of 504 S/m, suggesting that the filler was furtherdispersed. At high shear rate values (>10 s⁻¹) a decrease inconductivity is observed suggesting that some fiber breakup may occurupon application of high shear.

The same procedure was also followed to disperse doped P3HTnanostructures into a commercial polyurethane formulation (ParksPro-Finisher Polyurethane for Floors Clear). This formulation was thencoated using doctor blading to form a homogeneous wet film over apolyethylene terephthalate (PET) substrate with a thickness of 2 mils.The conductive paint was then allowed to dry and the surface resistivityof the conductive coating was measured using the Van der Pauw 4-pointprobe method. Surface resisitivity values of 1.4×10⁶ Ohm/sq weremeasured for a conductive filler content of 1 wt % (based on wetsample). Optimization of the network particle size, the dopant additive,the dispersion state and the concentration of filler can further reducethe resisitivity. For reference, the surface resisitivity of PETsubstrates ranges from 10¹²-10¹⁶ Ohm/sq depending on the relativehumidity. Therefore, enhancement of the surface electrical conductivityhas been demonstrated. Notably, the mechanical properties of the film(e.g. flexibility and adhesion) are similar to that of the originalpolyurethane carrier resin. FIG. 10 illustrates the flexibility of arepresentative sample applied to PET. The sample of FIG. 10 is measuredwith 4-point probe measurements according to the Van der Pauw method,for P3HT doped with iodine after self-assembly and dispersed at aconcentration of 1.2 wt % in a commercial polyurethane formulation. Theconductive films are flexible and retain the properties of purepolyurethane films because of the low filler loading fractions.

Effects of Doping on the Morphology of Conducting Polymer Nanostructures

We have designed model systems that will allow us to systematicallyanalyze and understand the intimate relationship between structure andproperties in conductive films prepared from CPs. We have used epoxyresins based on bisphenol A diglycidyl ether (BADGE) and commercialpolyurethane formulations as a model matrix materials, but expect theresults to also be relevant to other organic matrices used for vehicularfinishes. For the CP systems we have primarily used model systems ofalkyl substituted polythiophenes and polyfluorenes due to their richmorphological behavior. These polymers have molecular architectures thatallow them to exist in various morphological states (e.g. coils, fibers,networks, aggregates and gels) by tuning the self-assembly andaggregation. FIG. 11 presents SEM micrographs of exemplary CParchitectures, include P3HT in latex and nanofiber network form; PFO innanofiber network form, andpoly[(4,4′-bis(2-octyl)dithieno[3,2-b:2′3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen-2yl)thiazolo[5,4-d]thiazole)](PSOTT) in nanofiber network form. These results can be applied to thelarge diversity of CPs that have been synthesized within the last twodecades.

Because our interest is in conductive films, the materials are dopedwith oxidizing molecules. In this example, we explore two differentdopants, iodine and dodecylbenzene-sulfonic acid (DBSA). After theoxidation reaction, dopant molecules usually remain associated to thedoped CP forming a macromolecular salt. These two molecules allow us toevaluate the effect of dopant size on the self-assembly and morphologyof the CP nanostructures and on the properties of the resultingconductive coatings. We also use PEDOT:PSS dispersions as a benchmark CPmaterial to compare to our coatings. PEDOT:PSS is one of the mosteffective CP conductors. However, because it requires coating in itspure form, PEDOT:PSS films do not meet rigorous mechanical,environmental and adhesion specifications of vehicular finishes.

Doping of conjugated polymers has been demonstrated to lead to largeincreases in electronic conductivity. Charge conduction, whetheroccurring in a doped or un-doped state, requires percolation and is thusintimately tied to the morphology and nanostructure. CPs are idealmaterials for charge conduction because they self-assemble into avariety of nanostructures, including nanofibers and networks, when theyare dissolved in solvents of intermediate quality. This is driven by 1Dcrystallization due to π-π stacking interactions.

Doping of CPs is not common. Particularly, most CPs (e.g., P3HT) aretargeted for use as semiconducting compounds and doping or oxidation ofthe compounds is actively avoided. In the disclosed embodiments,however, the CPs are intentionally doped in order to improveconductivity. Undoped CPs integrated into composites as disclosed hereinwould not provide sufficient EME management materials to solve theproblems addressed by the disclosed embodiments.

Doping usually involves the oxidation of CPs leading to the injection ofpositive charge carriers (i.e. holes) that increase conductivity. Thesmall molecule iodine is frequently used to dope CP films. Doping isalso possible with larger acids that are soluble in organic solvents(e.g. DBSA).

One important effect of doping is that the dopant usually remainstightly associated to the polymer and forms an ionic complex that isanalogous to a macromolecular salt. Because the dopant remainsassociated to the polymer, it can obstruct the self-assembly of the CPbecause it can interfere with π-π stacking interactions. For example,when doping occurs prior to self-assembly (i.e. chains are doped in adissolved state), bulky dopants (e.g. DBSA) could prevent growth ofnanofibers and networks by intercalating between chains and instead leadto disordered aggregation of the CP (FIG. 1E).

These disordered structures lead to lower conductivities. To preventthis, nanofibers are grown via self-assembly in a desired solvent andtemperature in the un-doped state. Doping is then performed afterself-assembly so that the dopants only decorate the outside of thenanostructures and do not alter the internal morphology. Furthermore, itmay also be possible to substantially increase conductivity with smallerdopants (e.g. iodine) or by using lower amounts to avoid affecting thenano-scale morphology. Doping of CPs after the induction ofself-assembly allows retention of the original nanostructure and lead tohighly conductive materials.

The morphological effects of doping in systems of polythiophene CPsdispersed in aromatic solvents and in BADGE resins were studied. Dopingis induced in the aromatic solvents prior-to or after self-assembly ofnanofibers and networks by adding variable amounts of DBSA or iodine.Adjusting the level of supersaturation, via changes to temperature orsolvent quality, allows effective control of the morphology ofnanofibers and branched networks.

Percolation Behavior of CP Nanostructures

Composite materials incorporating conductive fillers typically showbehavior that is distinctive of percolating systems. Random percolationtheory, originally introduced in 1957 to describe the flow of fluidsthrough a porous medium, applies statistical analyses and models todescribe non-linear changes in macroscopic properties that occur whendispersed materials “percolate” or interconnect through a medium.Conductive nanocomposites, especially those containing additives withhigh aspect ratio, undergo steep non-linear increases in conductivitywith increasing concentration. In this example, we use existing modelsbased on random percolation theory to describe the concentrationdependence of the electrical properties of composite materialsincorporating CP additives with variable nanostructures (e.g. coils,nanofibers, networks or particles) as shown in FIG. 11. Percolationtheory and other models could be especially useful to rationally designnanostructures that maximize electrical conductivity and minimize therequired amount of additives and associated costs. These models can alsobe valuable tools for the formulation of coatings incorporating othertypes of conductive additives. The current processes that are used toformulate conductive coatings significantly benefit from the fundamentalunderstanding of the governing physical principles.

The conductivity of a material above its percolation threshold(φ_(crit)) can frequently be described with a simple power-law equation:σ=σ_(o)(φ−φ_(crit))^(t) where σ is the conductivity of the composite,σ_(o), is the conductivity of the pure filler material, φ is the volumefraction of filler and t is the critical exponent. In the context ofdesigning conductive finishes, it is desired to minimize the value ofφ_(crit) and to maximize the value of t so that smaller amounts ofadditives are required to achieve the desired properties. ExtensiveMonte Carlo simulations in two and three dimensions have been performedto theoretically estimate values for φ_(crit) and t for model systemshaving various shapes and states of orientation. Spheres typically havehigh percolation thresholds (e.g. 37 vol % for silver particles inBakelite) and low critical exponents (1.3-2) suggesting that they arenot effective shapes for additives. However, the percolation thresholdis also affected by particle size and generally increases with largerparticle radius. In contrast, randomly oriented conductive fibers havemuch lower percolation thresholds (e.g. 4.5 vol % for carbon fibers) andhigher critical exponents (t>3) due to their elongated shape. This shapeleads to large probabilities of fiber overlaps that help to create aconductive path. For fibers, φ_(crit) is again dependent on the fiberradius explaining why carbon nanotubes are such effective conductiveadditives. In general, nanostructures with high aspect ratios and highsurface-to-volume ratios (i.e. small size) result in improvedconductivity when randomly packed. However, fiber orientation effects,like those due to shear, can also increase the percolation threshold,lower the critical exponent and generally decrease conductivity.

Nanofiber networks are superior CP nanostructures in conductive coatingsbecause they are formed from elongated fiber subunits that lowerpercolation thresholds. In addition, they will also be less likely toundergo shear orientation due to their isotropic structures.

Percolation thresholds and critical exponents in networked nanomaterialshave not been studied in as much detail as spherical particles and fibersystems because there are fewer conductive additives available that havecontrollable network structures. Networks of CPs are spontaneouslyformed when nanofibers branch during the crystallization process due tolattice mismatch defects. The occurrence of defects, and thus thebranching frequency, can be manipulated by altering the supersaturationconditions of the polymer. It has been demonstrated that the networkmorphology can be modified to range from highly branched to looselybranched (even single fibers) by allowing self-assembly to proceed indifferent solvents or at different temperatures. The branching densitycan also be quantified by describing CP networks as fractal structureswhere the number of fibers (N) located in a sphere of radius (r) isdescribed by, N˜r^(D). The parameter D is commonly known as the fractaldimension and, for three-dimensional systems, its value ranges from 1(for un-branched fibers) to ˜3 (very dense solid-like networks). Thevalue of D is experimentally accessible has been measured in ourlaboratory for P3HT nanofiber networks (un-doped) with small angleneutron scattering (SANS). The average network size is also quantifiedvia electron microscopy (e.g. TEM, SEM). Impedance spectroscopy is usedto measure the conductivity of samples having different nanostructures,doping levels and concentrations.

FIG. 12 shows a representative Nyquist plot for a 1 wt % P3HT nanofibernetwork dispersion in BADGE epoxy resin that was doped with iodine afterinducing self-assembly. The impedance spectrum was modeled with aconstant phase element (CPE) model corresponding to the equivalentcircuit shown in FIG. 12. Impedance analysis allows isolation of all theresistances and capacitances that could affect the measurements in orderto ensure that accurate values of the bulk material conductivity andresistivity are obtained.

Flow-Induced Structural Transitions in Conductive Polymer Coatings

During common coating processes, materials are frequently subjected tohigh shear rates that can significantly affect the electronic propertiesof the final finish. In the case of spray coating, which is the mostcommon approach to apply aircraft finishes, the effective shear rate ofthe fluid being coated can range from 100 to 10,000 s⁻¹. Such high shearfields could result in significant alterations to the morphology andtherefore also affect the properties of the final coating. There are twoprimary mechanisms that could lead to deterioration of electricalconductivity due to flow effects: 1) flow-induced alignment ofconductive fillers and 2) shear induced morphological changes of theadditives (e.g. network fracture). In this example we discuss thefundamental principles leading to shear-induced transitions in oursystems.

Nanofiber networks of CPs will not undergo significant shear-alignmentbut can and will be fractured when local stress fields exceed a criticalvalue (τ_(c)). In contrast, nanofiber dispersions (i.e. un-branchedindividual fibers) undergo significant shear-alignment that will lead todeterioration of the electronic properties in the coatings.

Combined structure-property experiments can be used to systematicallystudy the effects of shear on the same systems that we have described inthe previous sections. We use a specially configured shear cell thatinterfaces with a commercial rheometer (Anton Paar MCR 301) tosimultaneously perform rheological and impedance spectroscopy analysis(i.e. rheodielectric tests) of CP nanostructures that are dispersed inepoxy resins (BADGE). FIG. 9 shows a schematic and a picture of thesetup along with an example of preliminary rheo-dielectric data for P3HTnanofiber networks.

This sample was doped with iodine after self-assembly and dispersed inun-cured BADGE epoxy (1 wt % P3HT). For reference, the conductivity ofthe neat BADGE resin is just 0.01 μS/m and the value does not changewith shear rate. In this experiment, the addition of just 1 wt % of theP3HT nanostructured networks resulted in an increase of more than 50,000times the conductivity of the epoxy. Notably, this is preliminary dataand the samples have not been optimized for doping levels, concentrationor morphological parameters. Thus, even larger improvements inconductivity could be expected from optimization.

More importantly, there is a complex dependence of the electricalconductivity with the applied shear-rate for this sample. At low shearrates (<0.04 s⁻¹) there is a steady increase in conductivity that couldresult from the re-organization of the filler material increasing theprobability of network contacts and improving percolation. Atintermediate shear rates (0.1-10 s⁻¹) there is a region of constantconductivity.

Finally, at high shear-rates (>10 s⁻¹) there is a steady decrease inconductivity as a function of shear that could indicate the onset ofnetwork breakup or orientation effects. The corresponding shear stresswhere the sharp decrease in conductivity occurs could be related to thecritical shear stress (τ_(c)) for network fracture. The conductivity inthis sample decreases by ˜60 times from its maximum value indicatingthat shear effects are significant.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of forming a composite incorporating networks of conductive polymer nanofibers, the method comprising the steps of: (a) providing a colloidal dispersion comprising a self-assembled network of nanofibers comprising a conjugated polymer; (b) doping the conjugated polymer with a chemical dopant to provide conductive polymers within the self-assembled network of the colloidal dispersion; and (c) dispersing the colloidal dispersion within a liquid matrix to provide a liquid composite comprising a network of conductive polymer nanofibers, wherein the liquid matrix is selected from the group consisting of a polymer and a polymer precursor.
 2. The method of claim 1 further comprising a step of solidifying the liquid composite to provide a solid composite comprising the network of conductive polymer nanofibers in a solid polymer matrix.
 3. The method of claim 1, wherein the colloidal dispersion is formed by temperature-induced self-assembly of the conjugated polymer in a solution.
 4. The method of claim 1, wherein the colloidal dispersion is a fluid colloidal dispersion.
 5. The method of claim 1, wherein the colloidal dispersion is prepared from the mechanical fracture of a gel.
 6. The method of claim 5, wherein the gel is an elastic organogel comprising the self-assembled network of the conjugated polymer.
 7. The method of claim 1, wherein the colloidal dispersion is formed by self-assembly through the gradual change of solvent composition selected from the group consisting of alkanes, aromatics, and halogenated organic molecules.
 8. The method of claim 1, wherein the conjugated polymer is a semiconducting polymer.
 9. The method of claim 1, wherein the conjugated polymer is selected from the group consisting of a polyalkylthiophene, a polydi-alkyl fluorene, a polydithienosilole, a polyphenylene, a poly(3,4-ethylenedioxythiophene), a poly(pyrrole), a polypyrene, a polypyridine, a poly(p-phenylene vinylene), a polycarbazole, a polyaniline, a polyindole or a copolymer of the polymers listed within this group.
 10. The method of claim 1, wherein the chemical dopant is selected from the group consisting of oxidizing agents including iodine, organic soluble sulfonic acids, water-soluble sulfonic acids, organic salts, and acidic polymers.
 11. The method of claim 1, wherein the liquid matrix is selected from the group consisting of a polymerizable resin, an oil-based paint, and an oil-based primer.
 12. The method of claim 1, wherein the step of dispersing the colloidal dispersion within the liquid matrix comprises dilution of the matrix and colloidal dispersion with a volatile organic solvent followed by concentration via solvent evaporation using heat or vacuum.
 13. The method of claim 1, wherein the step of dispersing the colloidal dispersion within the liquid matrix comprises sonication or mechanical blending.
 14. The method of claim 1, wherein the network of conductive polymers nanofibers comprises fibers having an individual length of from 50 nm to 5 microns.
 15. The method of claim 1, wherein the network of conductive polymers nanofibers comprises fibers having a cross-sectional dimension of from 5 nm to 200 nm.
 16. The method of claim 1, wherein the network of conductive polymers nanofibers comprises fibers having a plurality of branch points spaced between 200 nm to 5 microns apart.
 17. The method of claim 1, wherein the colloidal dispersion is from 1 micron to 1 mm in size. 